Nicene and Post-Nicene Fathers, Vol. II:City of God: Chapter 20

Now among the rites of Ceres, those Eleusinian rites are much famed which were in the highest repute among the Athenians, of which Varro offers no interpretation except with respect to corn, which Ceres discovered, and with respect to Proserpine, whom Ceres lost, Orcus having carried her away. And this Proserpine herself, he says, signifies the fecundity of seeds. But as this fecundity departed at a certain season, whilst the earth wore an aspect of sorrow through the consequent sterility, there arose an opinion that the daughter of Ceres, that is, fecundity itself, who was called Proserpine, from proserpere (to creep forth, to spring), had been carried away by Orcus, and detained among the inhabitants of the nether world; which circumstance was celebrated with public mourning. But since the same fecundity again returned, there arose joy because Proserpine had been given back by Orcus, and thus these rites were instituted. Then Varro adds, that many things are taught in the mysteries of Ceres which only refer to the discovery of fruits.